1. Field of the Invention
The present invention relates generally to an apparatus for writing information on an optical record medium such as an optical card by projecting a writing light beam onto the optical record medium and for reading the information out of the optical record medium by projecting a reading light beam, and more particularly to an optical information writing and reading apparatus comprising a first light source for emitting a writing light beam and a second light source for emitting a reading light beam and projecting these light beams simultaneously onto the same information track in an optical record medium.
2. Related Art Statement
The above mentioned optical information writing and reading apparatus has been known and is described in Japanese Patent Laid-open Publication Kokai Sho No. 2-61830. In the known optical information writing and reading apparatus, there are provided a first light source for emitting a writing light beam and a second light source for emitting a reading light beam separately from each other and these writing and reading light beams are projected onto the same information track of an optical card to form writing light spot and reading light spot on the same information track at positions separated from each other in a track direction in which the information track extends, and therefore it is possible to check a pit recorded by the writing light spot immediately after the recording. Such a checking operation is called a verifying operation. In this manner, it is possible to verify the recorded condition while the information is written on the optical card by the writing light beam. In another known optical information writing and reading apparatus in which only a single light beam is used for writing the information on an optical card and reading the information out of the optical record medium, in order to effect the verifying operation it is necessary to scan the optical record medium twice, i.e. a first scan for writing the information on the optical card and a second scan for reading the information just written on the optical card. Therefore, in the optical information writing and reading apparatus having the two light sources, the verifying operation can be performed at a high speed as compared with the apparatus comprising only one light source. That is to say, the effective writing speed of the former apparatus can be twice of the writing speed of the latter apparatus. Moreover, in the apparatus having the two light sources, the reading light beam emitted by the second light source may be maintained constant, so that it can be used to detect focusing and tracking errors, and therefore the tracking and focusing servo control can be carried out stably during the writing operation.
FIG. 1 is a schematic view showing the construction of an optical head of the known optical information writing and reading apparatus shown in FIG. 14 of the above mentioned Japanese Patent Laid-open Publication. The optical head comprises a semiconductor laser 1 for emitting a writing laser beam. A writing laser beam emitted by the semiconductor laser 1 is converted by a collimator lens 2 into a parallel laser beam having a substantially elliptical cross section. This laser beam is then converted by a shaping prism 3 for effecting a reduction only in a direction parallel to a longitudinal axis of the elliptical laser beam into a substantially circular laser beam. A diameter of the circular laser beam is then limited by a circular stop 4 such that a laser light spot having a given diameter is projected onto an optical card. The laser beam is made incident upon a polarizing beam splitter 5. Since this laser beam is composed of substantially S polarized component due to characteristics of the semiconductor laser 1, a substantial part of the laser beam is reflected by a reflection surface of the beam splitter 5 and is made incident upon an objective lens 6 along its optical axis. The laser beam is converged by the objective lens 6 and is projected onto an optical card 7 as a circular laser spot. Then a portion of the optical card 7 is locally heated by the laser spot and its optical property is changed thermally in a non-reversible manner. In this manner, a pit is formed on the optical card 7.
The optical head further comprises a light emitting diode 8 of an end face emitting type having a slit-like light emitting surface. The light emitting diode 8 constitutes a reading light source and emits a reading light beam. A reading light beam emitted by the light emitting diode 8 is converted by a collimator lens 9 into a substantially parallel light beam and is then made incident upon the polarizing beam splitter 5. Only P polarized component of the incident light beam is transmitted through the reflection surface of the beam splitter 5 and is made incident upon the objective lens 6 at a portion which is deviated from the optical axis of the objective lens. A principal light ray of the reading light beam is inclined with respect to the optical axis of the objective lens 5 such that the reading light beam is projected onto the optical card 7 at a point which is shifted from a point at which the writing light beam is projected. In this manner, an image of the slit-like light emitting surface of the light emitting diode 8 is formed on the optical card 7.
FIG. 2 is a schematic view showing a positional relation between a writing light spot 23 formed on the optical card 7 by the writing laser beam emitted by the semiconductor laser 1 and a reading light spot 24 formed by the reading light beam emitted by the light emitting diode 8. On the optical card 7, there are formed a plurality of parallel guide tracks 21 along a longitudinal direction of optical card, and information tracks 20 are formed between the guide tracks. The optical card 7 and the light beam spots 23 and 24 are relatively moved in directions a and b which are parallel with a track direction in which the guide tracks and information tracks extend. Usually the optical card 7 is moved in the track direction to effect the information writing and reading.
When a pulse signal which is modulated in accordance with the information to be written on the optical card 7 is supplied to the semiconductor laser 1, the semiconductor laser emits a pulsatory modulated laser beam and pits 22 are successively recorded in an information track 20 on the optical card as illustrated in FIG. 2. A distance between the writing light spot 23 and the reading light spot 24 on the optical card 7 can be adjusted by changing the above mentioned inclination angle between the writing light beam and the reading light beam during the manufacturing and adjusting operation of the optical head.
The information reading light beam emitted by the light emitting diode 8 is modulated by the guide tracks 21 and pits 22 and is reflected by the optical card 7. A reflected light beam is made incident upon the objective lens 6 and is converted thereby into a substantially parallel light beam. The parallel light beam is then made incident upon the polarizing beam splitter 5. Since this light beam is reflected by the optical card 7 in accordance with the reflection law, the reflected light beam is composed substantially of the P polarized component. Therefore, the light beam is transmitted through the reflection surface of the beam splitter 5. The light beam transmitted through the polarizing beam splitter 5 is reflected by a reflection mirror 14 and is then converged by a converging lens 15. A half of the light beam emanating from the converging lens 15 is transmitted through a half mirror 16 and is made incident upon a signal reproducing and tracking error detecting photodetector 17 and the remaining half is reflected by the half mirror and is made incident upon a focusing error detecting photodetector 18. As explained above the reading light beam is made incident upon the objective lens 6 at a point deviated from the optical axis, and thus the focusing error can be detected in accordance with the off-axis principle. This off-axis principle is shown in FIG. 8 of the above mentioned Japanese Patent Laid-open Publication. The focusing error detecting photodetector 18 comprises two light receiving elements arranged side by side in the track direction and a movement of an image of the reading light spot 24 projected on the optical card 7 due to the focusing error is detected to derive a focusing error signal.
FIG. 3 illustrates an image 23a of the writing laser beam spot 23 and an image 24a of the reading light beam spot 24, said images being formed on the first photodetector 17. The photodetector 17 comprises signal reproducing light receiving elements 32, 33 for receiving images of pits 22 on two adjacent information tracks and tracking error detecting light receiving elements 30, 31 for receiving an image of a track which situates between said two information tracks. It should be noted that the light receiving element 33 is arranged to detect the pits situating on the same information track on which the pit is written by the writing light beam. In FIG. 3, the reading light spot 24a is formed at a given position with respect to the light receiving elements 30 to 33 free from the tracking error. When there is a tracking error, an image of the guide track is deviated with respect to the tracking light receiving elements 30 and 31 in the direction perpendicular to the track direction so that the tracking error can be detected as a change in a difference between the light intensities received by these light receiving elements. That is to say, a tracking error signal can be produced by deriving a difference between output signals from the light receiving elements 30 and 31 by means of a differential amplifier. During the signal reading operation, only the light emitting diode 8 is energized and the pit information recorded in adjacent two information tracks can be simultaneously read out by the light receiving elements 32 and 33.
When the information signal is to be written on the optical card 7, both the semiconductor laser 1 and light emitting diode 8 are energized. When the optical card is moved in the direction a shown in FIG. 2, a pit 22 just formed by the writing beam spot 23 is moved toward the reading beam spot 24, so that when the relevant pit arrives at the position of the reading beam spot, there is produced a change in the amount of light received by the signal reading light receiving elements 33 of the first photodetector 17 to produce a reproduced signal. In this manner, when the optical card 7 is moved in the direction a, immediately after a pit has been written, its recorded condition can be verified by checking an output signal produced by the light receiving element 33 to perform the prompt verifying operation.
In the above explained known apparatus including the two light sources, when the optical card 7 is moved in the direction b, it is impossible to reproduce the pit information immediately after writing, because in this case the writing beam spot image 23a situates at an upstream position with respect to the reading light spot image 24a viewed in the moving direction b of the optical card.